Electronic device provided with a magnetic screening

ABSTRACT

An electronic device furnished with magnetic screening having a peak of resonant magnetic losses. The screening includes at least one inductive winding constituted by at least one segment of metallic wire wound around at least one assembly of magnetic filaments.

TECHNICAL FIELD

The present invention concerns an electronic device provided withmagnetic screening having a peak of resonant magnetic losses.

STATE OF THE PRIOR ART

The electronic devices of the known art may have their performanceimproved in a non-negligible manner by suppression of interference whichperturbs their operation.

A first solution for obtaining such a result consists of screening thecables which transmit this interference, the connections which allow itto enter the said electronic devices, and the circuits which generatethem, using the Faraday cage principle, that is, using a sheath or acasing which reflects the incident interferences towards the exterior.But in this solution, interferences emitted within the screening arethen returned to it.

A second solution consists of attenuating the interference ordefinitively suppressing it. Recourse is then had to materials havingpeaks of magnetic losses which attenuate the electromagnetic emissions.

At frequencies from 10 kHz to 3 GHz, soft magnetic materials(ferrimagnetic oxides, ferromagnetic metals) are particularly effective.But forming such materials into a flexible form to enable easyconformation, by constituting a sheath or a casing, excludes the use ofthe usual techniques for implementation of these materials: sintering offerromagnetic oxides, forging of ferromagnetic metals. The ferromagneticmaterials used are then often constituted by an elastomer loaded withmagnetic particles, thin iron strips, and wires, textiles, and knittedfabrics of metal.

To reach high frequencies (typically 1-1000 MHz), it is necessary todivide the characteristic geometrical dimensions (thickness, radius,etc.) of these materials so as not to be incommoded by their metalliccharacter (skin effect). The large relative resistivity of ferromagneticmetals in the amorphous or nanocrystalline state is particularlyfavourable for such frequencies.

The use of such materials in the form of a fine powder is unfavourablebecause of the generally higher frequency character of these powders,due to their shape anisotropy. A large variety of commercial softferromagnetic materials having a very reduced dimension (threads,ribbons, thin layers, plates, powders, etc.) are particularly suitablefor such applications.

Examples of the use of soft ferromagnetic materials having a very smalldimension are given in the documents referenced [1] and [2] at the endof the description, in which ferromagnetic particles are used,incorporated in an elastomer. As described in the document [3],ferromagnetic filaments sheathed in glass have a permeability parallelto the filament when the sign of the magneto-elastic couplingcoefficient (magnetostriction) is negative. They are then particularlyattractive to the extent that:

-   -   the metallic diameter is small compared with the wavelength,    -   their glass sheath confers electrical insulation,    -   their preparation by the method of the known art named “Taylor        Ulitovsky” is easy,    -   their mechanical characteristics enable processing by        technologies derived from textiles or from electrical cabling        (wrapping, weaving, knitting, etc.).

They can therefore be used in filter cables, for example low-pass, asdescribed in [4] and [5]. But in these documents, the position of themagnetic losses as a peak frequency is difficult and expensive tocontrol to the extent that such control requires an alloy change, aparameter modification of the manufacturing process, or post-preparationtreatments as described in the document [6].

Generally, the effectiveness of screening and filtering materials at agiven frequency is essentially conditioned by their magnetic losses,that is, by their imaginary permeability. In effect, magnetic materialsgenerally have an imaginary permeability which has a peak whose resonantfrequency and bandwidth are linked to their characteristics. Anadjustment of the resonant frequency is therefore possible by acting onthe nature and characteristics of the material. On the other hand, thewidth of the absorption line is always greater than 500 MHz inferromagnetic materials, which is troublesome when it is desired tofilter a frequency band narrower than 500 MHz and to allow the rest ofthe signal to pass.

The invention has as its object to propose a solution to this problem byenabling an adjustment of the magnetic loss peak width, typically up tovalues of the order of 1 MHz, easy regulation of the resonant frequency,and a significant increase of the maximum level of magnetic losses, inthe case where narrow bandwidths are sought with respect to conventionalmaterials, the filtering then being more effective, or else, at a givenefficiency, less filtering material being necessary.

SUMMARY OF THE INVENTION

The invention concerns an electronic device provided with magneticscreening having a peak of resonant magnetic losses, characterised inthat this screening comprises at least one inductive winding constitutedby at least one segment of metallic wire wound around at least one setof magnetic filaments.

The magnetic filaments may be sheathed in glass. The diameter of themetallic wire constituting the inductive winding may be comprisedbetween 5 μm and 1 mm. The length of this wire may be comprised between0,001 mm and 20 cm. The surface area of a turn may be comprised between0.01 mm² and 1 cm². The number of turns may be comprised between 0.5 and50. Each segment may comprise plural superposed windings of metallicwire. These windings may be formed in the opposite direction. The lengthover which a segment extends may be comprised between 0 and 50 mm. Thedistance between two neighbouring inductive segments may be comprisedbetween 0 and 50 mm. At least two inductive segments of differentcharacteristics may be combined. At least one textile thread withoutmagnetic or electrical properties may be used to hold the filaments inplace. A non-conductive thread which bears conductive segments may beused. A shaped conductive wire may be used, the fixation of the assemblyof conductive wire+magnetic filaments being effected by embedding in aresin, the conductive wire being sectioned at desired places in order toproduce the inductive segments. The assembly of conductive wire+magneticfilaments may be sectioned with grooves of depth equal to the wirediameter and for example over a length comprised between 0.1 and 50 mm.

In a first embodiment, the wire is wound on the core of a cable.

In a second embodiment, at least one layer of screening is disposed on acasing which generates at least one interference according to apolarisation. The screening wire is disposed in each layer so as toattenuate the interferences by placing it parallel to the magnetic fieldof an interference. The inductive segments may be periodically spaced,their distribution in each screening layer itself also being periodic. Abi-layer of screening may be used, a first layer dealing with a firstpolarisation and being transparent in the other, and a second layertreating a second polarisation, the screening wire of this second layerbeing regularly sectioned so as to cut off the reflector effect linkedto the conductivity of the magnetic filaments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the device of the invention.

FIG. 2 shows a wrapping installation.

FIG. 3 shows a first embodiment of the device according to theinvention, in the form of a screened cable.

FIGS. 4, 5A and 5B show a second embodiment of the device according tothe invention.

FIGS. 6 and 7 show curves of imaginary permeability as a function offrequency for two different examples of the device according to theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the device of the invention such as shown in FIG. 1, at least onesegment 10 of metallic wire constituting screening is wound, for exampleby a wrapping technique, on a wire 12 constituted by an assembly ofmagnetic filaments 13. Several segments of this type can therefore bewound, for example in a regular manner, on the wire 12.

The magnetic filaments 13 may be ferromagnetic filaments sheathed inglass, because of the previously cited advantages. Such ferromagneticfilaments, whose alloy is chosen so as to have a high permeabilityparallel to the wire 12, constitute the basic magnetic material. Thedesign rules for such filaments are given in the documents [7] and [8].

The segments 10 play the part of resonant inductive windings. Theirelectrical and geometrical characteristics, conductivity, diameter,length, and winding pitch regulate the position in frequency and thewidth of the peak of magnetic losses.

The conductivity of the wire 11 acts, for example, on the resistance ofthe inductive element: the greater the conductivity of this wire, thenarrower the peak. The resistivity (inverse of conductivity) variesstrongly as a function of the nature of the alloy used. Currently, wiresare found having a resistance per unit length of:

-   -   535.5 ohm/m (for example Isa-Chrom 80, diameter 50 μm, of the        IsabellenHuette Heusler GmbH Company),    -   169 Ohm/m (for example Resistherm, diameter 50 μm, of the        IsabellenHuette Heusler GmbH Company),    -   40 Ohm/cm (for example, 99.6% pure nickel, diameter 50 μm, of        the IsabellenHuette Heusler GmbH Company),    -   3.57 Ohm/cm (for example E-Kupfer copper, of 80 μm diameter, of        the of the IsabellenHuette Heusler GmbH Company).

The diameter of the wire 11 and the area of turns which it constitutes(number of turns×surface) act on the inductance of the winding: thefiner the wire and the greater the area of the turns, the more theresonant frequency of the winding decreases.

The wire diameter may for example be comprised between 5 μm and 1 mm.The length of wire 11 wound in each segment 10 may be comprised between0.001 mm and 20 cm. The surface of a turn may be comprised between 0.01mm² and 1 cm². The number of turns of each segment 10 may be comprisedbetween 0.5 and 50. Plural superposed windings, possibly made in theopposite direction, may constitute each segment 10. The length overwhich the winding of each segment extends may be comprised between 0 and50 mm. The distance between two neighbouring inductive segments 10 maybe comprised between 0 (quasi-contiguous segments) and 50 mm. Thedifferent inductive segments 10 may have different characteristics so asto create a spectrum of magnetic losses with two or more peaks ofimaginary permeability.

The technique used for constituting the windings of the conductivesegments 10 around the assembly of magnetic filaments 13 is related tothe technique of wrapping, which is a finishing technique much used fortextile threads. The parameters of wrapping (number of wrapping wires,sense of wrapping (S or Z), and pitch of wrapping on the wire, arevariables which enable inductive segments 10 to be realised withdifferent properties. Wrapping with, for example, two wires turning incontrary directions enables the inductive effect of the winding to beaugmented without extending its length and modifies the couplings of theinductive winding with the electric field parallel to the winding. Thefunction of keeping the filaments 13 in place may be performed by one ormore textile threads without magnetic or electrical properties. Aconsiderable torsion of the filaments 13 may be added so as to increasethe mechanical strength of the wire 12 (if a filament is cut, it doesnot reduce the overall strength of the wire because this is “jammed” bythe torsion).

In the wrapping installation, shown in FIG. 2, the wire 12 is obtainedby assembling plural magnetic filaments 13 coming from bobbins 20 afterpassage through two bushings 21 and 22. A receiving bobbin 23 having adirection of rotation 24 combined with a travelling comb (not shown),having a transverse movement of displacement 28, enables storage of awire 29 constituted by the said wire 12 after passage over a rotatingwheel 25 on which has been wound a wire 11 coming from a wrapping bobbin26 in rotation 27.

The conductive segments 10 may be formed before wrapping, for example byconstituting a non-conducting thread beforehand which bears theseconductive segments 10. This may for example be a textile thread onwhich a conductive paint has been disposed, this thread acting to wrapthe filaments 13. The segments 10 then act to keep the filaments 13 inplace.

Wrapping of the filaments 13 may also be performed using a conductivewire 11, shaping the wire 12 thus obtained, ensuring the fixation of theassembly for example by embedding it in a resin, and sectioning theconductive wire at the desired places in order to realise the inductivesegments 10.

To section the assembly of conductive wire 11+magnetic filaments 13,grooves may be made, of length comprised between 0.1 and 50 mm withlengths of magnetic filaments comprised between 0.1 and 50 mm.

In a first embodiment of the invention, the wire 12, obtained asdescribed above, is shaped as the wrapping or filtering material bywinding it on a core of a cable 32, as shown in FIG. 3. Following thespecification of attenuation of this cable 32 according to the frequencybands used, a screening wire 12 is produced whose conductive segments 10respond to this specification. The magnetic field generated by such acable 32 being orthoradial, the good utilisation geometry of thescreening wire 12 consists of turning the screening wire around the coreof the cable 32 so as to optimise the effect of the absorption of thefield by the wire 12. This wire 12 is therefore wound around theinternal structure 31 of this cable 32, which also comprises an externallayer 33.

In a second embodiment, illustrated in FIG. 4, a screening layer 40according to the invention is placed on a casing 41 undergoing aninterference in a direction 42 in a zone 43, to screen the latter byenabling the interference to be filtered. The screening wires 44,constituted by magnetic filaments 45, are disposed parallel to themagnetic field of the interference so as to attenuate it. Eightinductive segments 46, each of 3 turns 47, are wound around thesescreening wires 44. The density of these inductive segments 46, asshown, is low so as to preserve the clarity of the illustration. But itmay reach greater values (for example, by jointed patterns) in practice.As an example, in FIG. 4, the inductive segments 46 are periodicallyspaced on the screening wire 44, and their distribution in the screeninglayer 40 is also periodic. The segments 46 are placed at identicalabscissas×(parallel to the wire).

To increase the efficiency of the screening, plural layers of screeningmay be superposed. So, as illustrated in FIG. 5, a bi-layer screeningmay be used, as shown in FIG. 5B, to be placed on a casing 50 shown inFIG. 5A, in which the interference generated comprises components in thetwo polarisations of the magnetic field. The zones 51 and 52 arerespectively zones which are subject to interference following thepolarisation H1 and following the polarization H2. Each layer ofscreening deals with one polarization and is transparent in the other,and the other layer deals with the other polarisation. To obtain atransparent lower layer 40′, the screening wires 44′ of this layer areregularly sectioned, for example by means of groove 53, to cut off thereflector effect linked to the conductivity of the magnetic filaments45′. Such cuts may be realised at the same time as the cuts which aremade in the conductive wrapping wire used to produce the inductivesegments 46′. Such grooves 53 enable the transparency of the secondlayer 40′ to be ensured with respect to the electric field associatedwith the polarization H2.

Windings formed around magnetic materials have already been proposed,for example in the document [9].

The conductive character along the axis of the ferromagnetic filamentschanges the microwave frequency behaviour. As a result, there is anabsence of resonant interference on the permittivity. The possibilitiesof engineering the frequency response (bandwidth, position of absorptionpeak) offered by the conductive segments (wrapping pitch, number ofturns, etc.) enable realising absorption in the two polarisations overthe same frequency band.

EMBODIMENT EXAMPLES

In a first embodiment example, a set of ferromagnetic filaments is used,sheathed in glass by the Taylor Ulistovsky process. These filaments aremade of a soft commercial alloy with a diameter of 4 micrometers and theglass sheath 2 micrometers thick. As shown in FIG. 1, a copper wire iswound around the filaments 13 to form a sample whose permeability ismeasured by a microwave characterisation method. The copper wire is anenamelled copper wire 50 μm in diameter, which is periodically sectionedin order to form segments of conductive wire with a period of 6.2 mm,the length of the groove then formed being 0.6 mm. The area of the turnsis 1 mm². The length of these segments is 80 mm. The wrapping pitch is0.4 mm. FIG. 6 shows a curve 60 of imaginary permeability Pi of such asample without conductive segments and a curve 61 of magneticpermeability Pi of such a sample with conductive segments 61 accordingto the invention. The position of the resonant peak of permeability Pihas moved from 1.2 GHz to 0.2 GHz due to the inductive segments, whilethe bandwidth Δf at half height has moved from 768 MHz to 68 MHz. If theratio f/Δf is defined as the selectivity of the filter, this has beenmultiplied by 2, since it has passed from 1.5 to 3.

In a second embodiment example, to show how easily the screeningaccording to the invention may be used to filter a frequency band, asample prepared as previously is used, but with lengths of inductivesegments of 20 mm and a wrapping pitch of 1.5 mm. The position of thepeak of maximum losses has moved from 1.2 GHz (curve 62 withoutinductive segments) to 0.8 GHz (curve 63 with inductive segments) due tothe inductive segments, while the bandwidth at mid-height has gone from768 MHz to 400 MHz, and the selectivity of the filter has gone from 1.5to 2.

REFERENCES

[1] Documentation Tokin http://www.nec-tokin.com

[2] EP 1143458

[3] “High frequency losses of ferromagnetic wires near the gyromagneticresonance”. S. Deprot, A.-L. Adenot, F. Bertin, E. Herve and O. Acher(IEEE Trans. Magn, 37, 2404, 2001)

[4] WO 00/68959

[5] WO 00/31753

[6] “Frequency response engineering of CoFeNiBSi microwires in the GHzrange”. S. Deprot, A.-L. Adenot, F. Bertin, and O. Acher (J. Magn.Mater. 242, 247, 2002).

[7] “High frequency permeability of thin amorphous wires with variousanisotropic fields”. M. J. Malliavin, O. Acher, C. Boscher, F. Bertinand V. S. Larin (J. Magn. Magn. Mater. 196-197, 420, 1999)

[8] “Parallel permeability of ferromagnetic wires up to GHzfrequencies”. O. Acher, A. L. Adenot, S. Depcot. (J. Magn. Magn. Mater.249, 264, 2002)

[9] “Resonance phenomena in chiral and chiro-ferrite one-dimensionalmedia in the microwave band”. G. A. Kraftmakher, Yu K. Yazantsev. (J.Comm. Tech. Elec. 44, 1393-1402, 1999).

1. An electronic device comprising: a magnetic screening wire having apeak of resonant magnetic losses, said magnetic screening wireincluding, at least one assembly of magnetic filaments, and at least oneinductive winding including at least one segment of metallic wire woundaround said at least one assembly of magnetic filaments.
 2. A deviceaccording to claim 1, wherein the magnetic filaments are sheathed inglass.
 3. A device according to claim 1, wherein a diameter of themetallic wire is comprised between 5 μm and 1 mm, a length of the wireis comprised between 0.001 mm and 20 cm, a surface of a turn iscomprised between 0.01 mm₂ and 1 cm₂, and a number of turns is comprisedbetween 0.5 and
 50. 4. A device according to claim 1, wherein eachsegment comprises plural superposed windings of metallic wire.
 5. Adevice according to claim 4, wherein the plural windings are performedin opposite directions.
 6. A device according to claim 1, wherein eachsegment has a length comprised between 0 and 50 mm, a distance betweentwo neighboring segments being comprised between 0 and 50 mm.
 7. Adevice according to claim 1, wherein at least two inductive segments ofdifferent characteristics are combined.
 8. A device according to claim1, comprising at least one textile thread without magnetic or electricalproperties to ensure keeping the filaments in place.
 9. A deviceaccording to claim 1, comprising a non-conductive wire that carries theconductive segments.
 10. A device according to claim 1, whereinconductive wire is conformed, a fixation of the assembly of conductivewire and magnetic filaments being effected by embedding in a resin andsectioning the conductive wire at desired places to produce inductivesegments.
 11. A device according to claim 10, wherein the assembly ofconductive wire and magnetic filaments is sectioned with grooves.
 12. Adevice according to claim 11, wherein the grooves have a depth equal tothe diameter of the wire and over a length between 0.1 and 50 mm.
 13. Adevice according to claim 1, wherein the magnetic screening wire iswound on a core of a cable.
 14. A device according to claim 1, whereinat least one layer of the magnetic screening wire is disposed on acasing that generates at least one interference according to apolarization, in which the magnetic screening wire is structured in eachlayer so as to attenuate an interference by placing the magneticscreening wire parallel to the magnetic field of the interference.
 15. Adevice according to claim 14, further comprising: inductive segments ofthe at least one inductive winding spaced periodically on the assemblyof magnetic filaments, a distribution of the inductive segments beingperiodic.
 16. A device according to claim 14 comprising first and secondscreening layers.
 17. A device according to claim 16, wherein the firstscreening layer deals with a first polarization and is transparent inthe other, and the second screening layer deals with a secondpolarization, the screening wire of the second layer being regularlysectioned so as to cut off a reflector effect linked to conductivity ofthe magnetic filaments.